Quantum key distribution information leakage due to backflashes in single photon avalanche photodiodes

ABSTRACT

A quantum cryptography apparatus and system includes a photon emitter, a photon receiver, a first photodetector, a second photodetector, a first polarization optic, and a second polarization optic. The photon emitter is configured to emit a photon at a wavelength. The photon receiver is coupled to the photon emitter by at least one quantum channel. The photon receiver includes the first polarization optic configured to output the emitted photon in a polarization state. The first photodetector is configured to detect the emitted photon from the output of the first polarization optic. The second photodetector is configured to detect a backflash from the first photodetector. The second polarization optic is between the first photodetector and the second photodetector. The second photodetector and the second polarization optic are configured to internally calibrate the photon receiver.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.15/702,085, filed Sep. 12, 2017, which is hereby incorporated herein inits entirety by reference.

BACKGROUND Field

The present disclosure relates to quantum cryptography apparatuses andsystems. More specifically, embodiments of the present disclosure relateto quantum cryptography apparatuses and systems to detect andcharacterize information leakage due to backflashes in single photonphotodetectors.

Background

Quantum key distribution (QKD) promises a theoretically unbreakablecryptosystem by employing the probabilistic nature of quantummeasurement over mutually unbiased bases. Nevertheless, QKD systemspossess security vulnerabilities due to engineering, technical, andtechnological imperfections in practical implementations. For example,single photon photodetectors used in QKD systems could be a source ofinformation leakage due to the emission of unaccounted photons orsecondary emissions, deemed backflashes, that occur after the incidentor main information-carrying photons impinge and are detected by aphotodetector, in particular, an avalanche photodiode (APD).

SUMMARY

In some embodiments, a quantum cryptography apparatus includes a photonemitter configured to emit a photon at a wavelength, a photon receivercoupled to the photon emitter by at least one quantum channel andincluding a first polarization optic configured to output a polarizationstate of the emitted photon, a first photodetector configured to detectthe emitted photon from the output of the first polarization optic, asecond photodetector configured to detect a backflash from the firstphotodetector, and a second polarization optic between the firstphotodetector and the second photodetector. In some embodiments, thesecond polarization optic is configured to detect a polarizationdependence of the backflash from the first photodetector.

In some embodiments, the photon receiver includes two arms, each armassociated with a BB84 basis. In some embodiments, the photon receiverincludes a non-polarizing 50:50 beamsplitter that couples the two arms.In some embodiments, the photon receiver comprises two arms, each armassociated with a BB84 basis, and a non-polarizing 50:50 beamsplitterthat couples the two arms.

In some embodiments, the at least one quantum channel is a free spacechannel. In some embodiments, the wavelength of the emitted photon is400 nm to 1100 nm. In some embodiments, the at least one quantum channelis a free space channel and the wavelength of the emitted photon is 400nm to 1100 nm.

In some embodiments, the at least one quantum channel is a fiber opticchannel. In some embodiments, the wavelength of the emitted photon is1100 nm to 1600 nm. In some embodiments, the at least one quantumchannel is a fiber optic channel and the wavelength of the emittedphoton is 1100 nm to 1600 nm.

In some embodiments, the second photodetector is a four channel singlephoton counting avalanche photodiode (APD) array. In some embodiments,the second photodetector is configured to simultaneously detect thepolarization dependence of the backflash from the first photodetector.In some embodiments, the second photodetector is a four channel singlephoton counting avalanche photodiode (APD) array and configured tosimultaneously detect the polarization dependence of the backflash fromthe first photodetector.

In some embodiments, the first photodetector is a four channel singlephoton counting avalanche photodiode (APD) array. In some embodiments,the first photodetector is configured to simultaneously detect thepolarization state of the emitted photon from the photon emitter. Insome embodiments, the first photodetector is a four channel singlephoton counting avalanche photodiode (APD) array and configured tosimultaneously detect the polarization state of the emitted photon fromthe photon emitter.

In some embodiments, the second polarization optic comprises anadjustable linear polarizer. In some embodiments, the secondpolarization optic can include a plurality of second polarizationoptics. In some embodiments, the first polarization optic can include aplurality of first polarization optics. In some embodiments, the emittedphoton from the photon emitter is circularly polarized. In someembodiments, the emitted photon from the photon emitter is linearlypolarized. In some embodiments, the emitted photon from the photonemitter is elliptically polarized. In some embodiments, the emittedphoton from the photon emitter is randomly polarized.

In some embodiments, the apparatus includes a calibration of the quantumkey distribution system based on the polarization dependence of thebackflash. In some embodiments, the apparatus includes a trigger for analarm subsystem based on the polarization dependence of the backflash.In some embodiments, the apparatus includes a deterrent for subsequentmeasurement of the backflash based on the polarization dependence of thebackflash.

In some embodiments, a quantum key distribution system forcharacterizing backflashes includes a photon emitter configured to emita photon at a wavelength, a photon receiver coupled to the photonemitter by at least one quantum channel and including a firstpolarization optic configured to output a polarization state of theemitted photon, a first photodetector configured to detect the photonemitted from the output of the first polarization optic, a secondphotodetector configured to detect a backflash from the firstphotodetector, a second polarization optic between the firstphotodetector and the second photodetector. In some embodiments, thesecond polarization optic is configured to detect a polarizationdependence of the backflash from the first photodetector.

In some embodiments, a data acquisition subsystem is coupled to thesecond photodetector. In some embodiments, the data acquisitionsubsystem characterizes the polarization dependence of the backflash. Insome embodiments, the data acquisition subsystem includes an alarmsubsystem based on the polarization dependence of the backflash. In someembodiments, the data acquisition subsystem is configured to decrypt aquantum key distribution between the photon emitter and the photonreceiver.

In some embodiments, the system includes a calibration of the quantumkey distribution system based on the polarization dependence of thebackflash. In some embodiments, the system includes a trigger for analarm subsystem based on the polarization dependence of the backflash.In some embodiments, the system includes a deterrent for subsequentmeasurement of the backflash based on the polarization dependence of thebackflash.

In some embodiments, a method for characterizing backflashes in aquantum key distribution system includes emitting a photon from a photonemitter at a wavelength, transferring the emitted photon by at least onequantum channel to a photon receiver, receiving the emitted photonthrough a first polarization optic with a first photodetector, detectinga backflash from the first photodetector with a second photodetector,and characterizing a polarization dependence of the backflash through asecond polarization optic.

In some embodiments, the method includes calibrating the quantum keydistribution system based on the polarization dependence of thebackflash. In some embodiments, the method includes triggering an alarmsubsystem based on the polarization dependence of the backflash. In someembodiments, the method includes deterring subsequent measurement of thebackflash based on the polarization dependence of the backflash.

In some embodiments, deterring subsequent measurement of the backflashincludes implementing an optical isolator, an optical circulator, anoptical modulator, and an optical filter, or some combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are incorporated herein and form a part of thespecification.

FIG. 1 illustrates a schematic diagram of a quantum cryptographyapparatus, according to some embodiments.

FIG. 2 illustrates a schematic diagram of a photon receiver, accordingto some embodiments.

FIG. 3 illustrates a flow diagram for characterizing a backflash,according to some embodiments.

FIG. 4 illustrates a schematic diagram of a system for characterizingbackflashes, according to some embodiments.

FIG. 5 illustrates a plot of the polarization dependence of backflashes,according to some embodiments.

FIG. 6 illustrates a schematic diagram of a QKD system, according tosome embodiments.

In the drawings, like reference numbers generally indicate identical orsimilar elements. Additionally, generally, the left-most digit(s) of areference number identifies the drawing in which the reference numberfirst appears.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described in detail withreference to embodiments thereof as illustrated in the accompanyingdrawings. References to “one embodiment,” “an embodiment,” “someembodiments,” etc., indicate that the embodiment(s) described mayinclude a particular feature, structure, or characteristic, but everyembodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is within the knowledge of one skilled in the art toaffect such feature, structure, or characteristic in connection withother embodiments whether or not explicitly described.

The following examples are illustrative, but not limiting, of theembodiments of this disclosure. Other suitable modifications andadaptations of the variety of conditions and parameters normallyencountered in the field, and which would be apparent to those skilledin the relevant art(s), are within the spirit and scope of thedisclosure.

Quantum key distribution (QKD) is a method for sharing of secretcryptographic keys between two parties with an unprecedented level ofsecurity. A sender (sometimes referred to herein as “Alice”) encodes asecret key in the form of quantum states and transmits them to areceiver (sometimes referred to herein as “Bob”), who performs quantummeasurements to obtain the key. Alice and Bob then use the shared secretkey to send encrypted messages to each other. Ideal secrecy is achievedwhen the probability distribution of all possible unencrypted messagesis equal to the probability distribution of all possible encryptedmessages. This level of security is assured by the laws of quantummechanics and does not depend on technological resources available to aneavesdropper (sometimes referred to herein as “Eve”), provided that theQKD implementation does not deviate from its theoretical model. However,the security of practical systems depends on their deviceimplementations. Deviations of QKD devices from their theoretical modelcan be exploited by an eavesdropper though side-channel or back-doorattacks.

Practical QKD implementations utilize protocols, for example, BB84, withphotons as the quantum state carriers that can utilize existinglong-distance fiber communication networks. Recently, free-space QKDsystems have been explored for their potential global scaleapplications, eliminating fibers and utilizing optical communicationfrom ground to low-orbital satellites. Such implementations typicallyuse single photon avalanche photodiodes (SPADs) to measure quantum basisstates. Systems that use a free-space quantum channel, generally 400 nmto 1100 nm wavelengths, typically employ silicon (Si) SPADs, whereassystems that use a fiber optic quantum channel, generally 1100 nm to1600 nm wavelengths, in telecommunications networks use indium galliumarsenide (InGaAs) SPADs. Avalanches of charge carriers in both Si andInGaAs SPADs are known to be accompanied by photon back reflections orsecondary emissions, known generally as backflashes, due toelectron-hole recombination in the SPADs. Such backlash photons couldcarry information regarding the qubits sent by Alice to Bob. Since it isimpossible to directly measure a quantum state without collapsing it,Eve cannot intercept states mid-transmission to acquire the keys withoutdestroying the quantum state. However, Eve may be able to utilizebackflashes to tap into the quantum channel shared between Alice and Boband go undetected, since the quantum bit error rate (QBER) isindependent of backflash photons.

Backflashes are secondary photons or secondary emissions caused byelectron-hole recombination and carrier relaxation (e.g., direct,phonon-assisted, or Bremsstrahlung) in a photodiode that occur whenphotons impinge a semiconductor material. Avalanche photodiodes (APDs)utilize a semiconductor p-n junction and when the reverse bias voltageof the p-n junction is raised to a breakdown voltage, absorption by asingle photon generates carriers in the conduction band which triggersan avalanche process. The avalanche process generates a measureablecurrent which can be used to register the detection time of a singlephoton. Backflashes may be either absorbed in a quiescent region of thesemiconductor, triggering new avalanches, or may be coupled back intothe quantum channel and tapped by Eve who can potentially deduce thestates of the original information-carrying photons. Since the creationof backflashes is a random process, Alice and Bob have no knowledge ofor control over backflashes that escape Bob's receiver. As backflashesexit Bob's receiver, for example, a BB84 receiver, they acquire distinctpolarization states by transmitting through optical elements in Bob'sreceiver. Thus, a backflash can obtain a unique polarization state basedon the polarization optics and scheme in the photon receiver.

FIG. 1 illustrates quantum cryptography apparatus 100, according to anembodiment. Quantum cryptography apparatus 100 can include photonemitter 102, photon receiver 104, and quantum channel 120. Alice createsrandomly polarized photons with photon emitter 102 and sends emittedphoton 116 with polarization state 122 along quantum channel 120 to Bobwho receives and measures emitted photon 116 with polarization state 122with photon receiver 104. In some embodiments, quantum channel 120 canbe free-space channel 136 and emitted photon 116 can have a wavelengthof wavelength of 400 nm to 1100 nm. As shown in FIG. 1, for example,quantum channel 120 can include first collimator 128 and secondcollimator 130. In some embodiments, quantum channel 120 can be fiberoptic channel 138 and emitted photon 116 can have a wavelength of 1100nm to 1600 nm. As shown in FIG. 1, for example, photon emitter 102 caninclude emitter fiber optic port 140 and photon receiver 104 can includereceiver fiber optic port 141 in some embodiments, as shown in FIG. 1,quantum channel 120 can be free-space channel 136, photon emitter 102can include emitter fiber optic port 140 coupled to first collimator128, and photon receiver 104 can include receiver fiber optic port 142coupled to second collimator 130. In some embodiments, emitted photon116 with polarization state 122 can be circularly polarized 126.

Photon receiver 104 may include first polarization optic 108, firstphotodetector 112, and receiver input port 132. Emitted photon 116 withpolarization state 122 transmits through receiver input port 132 andfirst polarization optic 108. First polarization optic 108 is configuredto output polarization state 122 of emitted photon 116. For example, asshown in FIG. 2, first polarization optic 108 can be non-polarizing50:50 beamsplitter 202. In some embodiments, first polarization optic108 can be a plurality of first polarization optics including, forexample, non-polarizing 50:50 beamsplitter 202, half-wave plate (π/8)222, first polarizing beamsplitter 224, and/or second polarizingbeamsplitter 226, or any combination thereof. First photodetector 112 isconfigured to detect emitted photon 116 from the output of firstpolarization optic 108. In some embodiments, first photodetector 112 isfour channel single photon APD array 212. For example, as shown in FIG.2, four channel single photon APD array 212 may be configured tosimultaneously detect polarization state 122 of emitted photon 116 fromphoton emitter 102. As shown in FIG. 1, first photodetector 112 maygenerate backflash 118, which can be detected by Bob (internally) or Eve(externally), traveling back out of first polarization optic 108,receiver input port 132, receiver fiber optic port 142, secondcollimator 130, and second photodetector 114 in a directionanti-parallel to fiber optic channel 138. In some embodiments, photonreceiver 104 can be BB84 receiver 200.

In some embodiments, quantum cryptography apparatus 100 can furtherinclude Eve with eavesdropping receiver 106. Eavesdropping receiver 106can include second polarization optic 110, second photodetector 114, andoptical circulator 134. As shown in FIG. 1, for example, Eve can utilizeoptical circulator 134 to tap into quantum channel 120 and siphon offbackflash 118 without detection by Alice or Bob, and without disturbingpolarization state 122 of emitted photon 116. Eve can then detectbackflash 118 with second photodetector 114 and detect polarizationdependence 124 of backflash 118 by using second polarization optic 110to deduce the original polarization state 122 of emitted photon 116. Insome embodiments, eavesdropping receiver 106 may be configured to detectbackflash 118 from first photodetector 112 with second polarizationoptic 110. Second polarization optic 110 may be configured to outputpolarization dependence 124 of backflash 118. In some embodiments,second polarization optic 110 can be a plurality of first polarizationoptics 108 including, for example, non-polarizing 50:50 beamsplitter202, half-wave plate (π/8) 222, first polarizing beamsplitter 224,second polarizing beamsplitter 226, and/or optical circulator 134, orany combination thereof. Second photodetector 114 may be configured todetect backflash 118 from the output of second polarization optic 110.In some embodiments, second photodetector 114 is four channel singlephoton APD array 212. In some embodiments, optical circulator 134 can bean optical isolator, an optical modulator, and/or an optical filter, orsome combination thereof. Similar to photon receiver 104, in someembodiments, eavesdropping receiver 106 can mimic the same or similaroptical arrangement and scheme as photon receiver 104, for example, BB84receiver 200.

As will be appreciated by persons skilled in the relevant art(s), BB84is a QKD scheme developed by Bennett and Brassard in 1984, and was thefirst quantum cryptography protocol. In some embodiments, BB84 can beimplemented using photon polarization, photon phase, or photon frequencyencoding. When modeled as a two-state quantum system, photonpolarization includes two quantum states that form a complete orthogonalbasis spanning the two-dimensional Hilbert space. A common pair of basisstates is horizontal |H

=|0

and vertical |V

=|1

, which are orthogonal to each other and form polarization basis HV.Through superposition, two additional orthogonal states can be createdand deemed antidiagonal |A

=|−

and diagonal |D

=|+

, which are orthogonal to each other and form polarization basis AD, butare non-orthogonal to polarization basis HV. Thus, together bases HV andAD give the following four qubit states:

$\left. {{\left. \left. {\left. \left. {\left. {\left. {\left. {\left. {\left. {\left. {\left. {\left. {❘H} \right\rangle = {❘\psi_{00}}} \right\rangle = {❘0}} \right\rangle,{❘V}} \right\rangle = {❘\psi_{10}}} \right\rangle = {❘1}} \right\rangle,{{\left. {{\left. \left. {\left. \left. {\left. {\left. {❘A} \right\rangle = {❘\psi_{01}}} \right\rangle = {{{\frac{1}{\sqrt{2}}\left( {❘H} \right\rangle} -}❘V}} \right\rangle \right) = {{{\frac{1}{\sqrt{2}}\left( {❘0} \right\rangle} -}❘1}} \right\rangle \right) \equiv}❘ -} \right\rangle,}❘D}} \right\rangle = {❘\psi_{11}}} \right\rangle = {{{\frac{1}{\sqrt{2}}\left( {❘H} \right\rangle} +}❘V}} \right\rangle \right) = {{{\frac{1}{\sqrt{2}}\left( {❘0} \right\rangle} +}❘1}} \right\rangle \right) \equiv}❘ +} \right\rangle,$Upon measurement, |H

and |A

correspond to bit 0, and |V

and |D

correspond to bit 1. In order to send randomly polarized photons, Alicerandomly chooses a polarization basis, either HV or AD, and records thisbasis information. Alice then creates a photon with a randompolarization in that selected basis, and records the polarization stateof the emitted photon and the associated bit value before sending to Bobover the quantum channel. Thus, each photon Alice creates has a randompolarization state with a 25% probability of being either |H

, |V

, |A

, or |D

. Likewise, Bob randomly chooses a polarization basis, either HV or AD,records this basis information, and measures the random polarizationstate in the form of a corresponding bit, either 0 or 1, for eachreceived photon.

In some embodiments, Alice creates a photon with random polarization byusing four photon sources, for example, diode lasers, each associatedwith one of the four polarization states 122 used in the BB84 protocol.In some embodiments, Alice creates a photon with random polarization byusing a single photon source and polarization optics, similar to firstpolarization optic 108, to create four polarization paths, eachassociated with one of the four polarization states 122 used in the BB84protocol. In some embodiments, the polarized emitted photon 116 is thentransmitted to Bob along a fiber optic channel 138. In some embodiments,the polarized emitted photon 116 is then transmitted to Bob alongfree-space channel 136.

FIG. 2 illustrates BB84 receiver 200, according to an embodiment. BB84receiver 200 is one example type of photon receiver 104 that can beutilized in quantum cryptography apparatus 100. BB84 receiver 200 caninclude first polarization optic 108 and first photodetector 112. Asshown in FIG. 2, for example, first polarization optic 108 can includenon-polarizing 50:50 beamsplitter 202, first arm 204, and second arm206. In some embodiments, first arm 204 corresponds to HV basis 208, andsecond arm 206 corresponds to AD basis 210. Non-polarizing 50:50beamsplitter 202 connects first arm 204 and second arm 206, and performsthe step of randomly choosing the basis for Bob by transmitting emittedphoton 116 into first arm 204 (HV basis 208) or second arm 206 (AD basis210) with equal probability. As shown in FIG. 2, for example, firstphotodetector 112 can be four-channel APD array 212, which includesfirst APD 214, second APD 216, third APD 218, and fourth API) 220. HVbasis 208 can include first polarizing beamsplitter 224, first APD 214,and second APD 216. AD basis 210 can include half-wave plate (π/8) 222,second polarizing beamsplitter 226, third APD 218, and fourth APD 220.

First and second polarizing beamsplitters 224, 226 split emitted photon116 according to whether emitted photon 116 is p-polarized (parallel tobeamsplitter) or s-polarized (orthogonal to beamsplitter). If emittedphoton 116 transmits through non-polarizing 50:50 beamsplitter 202, itenters first arm 204 and is detected by either first APD 214 or secondAPD 216 in HV basis 208. For example, emitted photon 116 that haspolarization state 122 of |H

transmits through first polarizing beamsplitter 224 and impinges firstAPD 214 and registers as bit 0 in first photodetector output 228.Similarly, in HV basis 208, emitted photon 116 that has polarizationstate 122 of |V

is split and reflected by first polarizing beamsplitter 224 and impingessecond APD 216 and registers as bit 1 in first photodetector output 228.

Alternatively, if emitted photon 116 is reflected by non-polarizing50:50 beamsplitter 202, it enters second arm 206 and is detected byeither third APD 218 or fourth APD 220 in AD basis 210. As emittedphoton 116 enters second arm 206, it transmits through half-wave plate(π/8) 222, which when rotated π/8 (i.e., 22.5°) with respect to thehorizontal, has the effect of rotating the linear polarization ofemitted photon 116 by π/4 (i.e., 45°). For example, after being rotatedby half-wave plate (π/8) 222, emitted photon 116 that has polarizationstate 122 of |D

transmits through second polarizing beamsplitter 226 and impinges fourthAPD 220 and registers as bit 1 in first photodetector output 228.Similarly, in AD basis 210, after being rotated by half-wave plate (π/8)222, emitted photon 116 that has polarization state 122 of |A

is split and reflected by first polarizing beamsplitter 224 and impingesthird APD 218 and registers as bit 0 in first photodetector output 228.

Due to the design of BB84 receiver 200, any backflash 118 created byfour-channel APD array 212 acquires specific polarization information asbackflash 118 transmits back through first polarization optic 108 andexits receiver input port 132. There is no direct coherence, classicalor quantum, between emitted photon 116 from Alice and backflash 118 fromBob, since emitted photon 116 is destroyed (e.g., absorbed, annihilated,recombined, etc.) in the measurement process with first photodetector112, and backflash 118 created is an ordinary unpolarized photon. Forexample, when backflash 118 generates in and exits first APD 214, onlybackflash 118 with polarization state 122 of |H

transmits back through first polarizing beamsplitter 224, and any otherbackflash 118 with a different polarization state 122 is reflected andlost. When backflash 118 with polarization state 122 of |H

from first APD 214 reaches non-polarizing 50:50 beamsplitter 202,backflash 118 is either transmitted and exits receiver input port 132 oris reflected and lost with equal probability. A similar procedure occursfor backflash 118 when generated by the other respective APDs 216, 218,220. Since each of Bob's APDs 214, 216, 218, 220 will produce backflash118 with one of four unique polarization states 122, Eve is able toaccurately reconstruct the results of Bob's measurements.

In some embodiments, quantum channel 120 is fiber optic channel 138.Polarization state 122 of emitted photon 116 can be transformed asemitted photon 116 travels along fiber optic channel 138. In someembodiments, third polarization optic 230 is utilized, for example, aset of waveplates, to transform polarization state 122 of emitted photon116 back to the original state sent by Alice. In some embodiments, BB84receiver 200 can include receiver fiber optic port 142.

FIG. 3 illustrates flow diagram 300 for characterizing backflash 118,according to an embodiment. It is to be appreciated that not all stepsin FIG. 3 may be needed to perform the disclosure provided herein.Further, some of the steps may be performed simultaneously, or in adifferent order than shown in FIG. 3. Flow diagram 300 shall bedescribed with reference to FIGS. 1 and 2. However, flow diagram 300 isnot limited to those example embodiments.

In 302, as shown in the example of FIG. 1, Alice creates randomlypolarized emitted photon 116 and sends emitted photon 116 to Bob overquantum channel 120. Alice creates randomly polarized photons withphoton emitter 102 and sends emitted photon 116 with polarization state122 along quantum channel 120 to Bob. In some embodiments, quantumchannel 120 can be free-space channel 136 and emitted photon 116 canhave a wavelength of wavelength of 400 nm to 1100 nm. In someembodiments, quantum channel 120 can be fiber optic channel 138 andemitted photon 116 can have a wavelength of 1100 nm to 1600 nm. As shownin FIG. 1, for example, photon emitter 102 can include emitter fiberoptic port 140. In some embodiments, as shown in FIG. 1, quantum channel120 can be free-space channel 136 and photon emitter 102 can includeemitter fiber optic port 140 coupled to first collimator 128. In someembodiments, emitted photon 116 with polarization state 122 can becircularly polarized 126.

Alternatively, as shown in the example of FIG. 6, Alice can createrandomly polarized emitted photon 616 and send emitted photon 616 to Bobover quantum channel 620. Alice creates randomly polarized photons withphoton emitter 602 and sends emitted photon 616 with polarization state622 along quantum channel 620 to Bob. In some embodiments, quantumchannel 620 can be free-space channel 648 and emitted photon 616 canhave a wavelength of wavelength of 400 nm to 1100 nm. In someembodiments, quantum channel 620 can be fiber optic channel 650 andemitted photon 616 can have a wavelength of 1100 nm to 1600 nm. In someembodiments, emitted photon 616 with polarization state 622 can becircularly polarized 652.

In 304, as shown in the example of FIG. 1, Bob receives emitted photon116 from Alice and measures polarization state 122 of emitted photon 116with photon receiver 104, which can include first polarization optic 108and first photodetector 112. Emitted photon 116 with polarization state122 transmits through receiver input port 132 and first polarizationoptic 108. First polarization optic 108 is configured to outputpolarization state 122 of emitted photon 116. For example, as shown inFIG. 2, first polarization optic 108 can be non-polarizing 50:50beamsplitter 202. In some embodiments, first polarization optic 108 canbe a plurality of first polarization optics including, for example,non-polarizing 50:50 beamsplitter 202, half-wave plate (π/8) 222, firstpolarizing beamsplitter 224, and/or second polarizing beamsplitter 226,or any combination thereof. First photodetector 112 is configured todetect emitted photon 116 from the output of first polarization optic108. In some embodiments, first photodetector 112 is four channel singlephoton APD array 212. For example, as shown in FIG. 2, four channelsingle photon APD array 212 may be configured to simultaneously detectpolarization state 122 of emitted photon 116 from photon emitter 102. Asshown in FIG. 1, first photodetector 112 may generate backflash 118,which can be detected by Bob (internally) or Eve (externally), travelingback out of first polarization optic 108, receiver input port 132,receiver fiber optic port 142, and second collimator 130 in a directionanti-parallel to emitted photon 116. In some embodiments, photonreceiver 104 can be BB84 receiver 200. In some embodiments,eavesdropping receiver 106 can be BB84 receiver 200. In someembodiments, as shown in FIG. 1, for example, photon receiver 104 caninclude receiver fiber optic port 142. In some embodiments,eavesdropping receiver 106 can include receiver fiber optic port 142. Insome embodiments, as shown in FIG. 1, quantum channel 120 can befree-space channel 136 and photon receiver 104 can include receiverfiber optic port 142 coupled to second collimator 130.

Alternatively, as shown in the example of FIG. 6, Bob receives andmeasures emitted photon 616 with polarization state 622 with photonreceiver 604. Photon receiver 604 may include BB84 receiver 200,backflash receiver 606, data acquisition (DAQ) subsystem 634, and firstinput port 654. In some embodiments, BB84 receiver 200 may include firstpolarization optic 108, first photodetector 112, and receiver input port132. Emitted photon 616 with polarization state 622 transmits throughreceiver input port 132 and first polarization optic 108. Firstpolarization optic 108 is configured to output polarization state 622 ofemitted photon 616. For example, first polarization optic 108 can benon-polarizing 50:50 beamsplitter 202. In some embodiments, firstpolarization optic 108 can be a plurality of first polarization opticsincluding, for example, non-polarizing 50:50 beamsplitter 202, half-waveplate (π/8) 222, first polarizing beamsplitter 224, and/or secondpolarizing beamsplitter 226, or any combination thereof. Firstphotodetector 112 is configured to detect emitted photon 616 from theoutput of first polarization optic 108. In some embodiments, firstphotodetector 112 is four channel single photon APD array 212. Forexample, four channel single photon APD array 212 is configured tosimultaneously detect polarization state 622 of emitted photon 616 fromphoton emitter 602. As shown in FIG. 6, BB84 receiver 200 generatesbackflash 618 traveling back out of BB84 receiver 200 in a directionanti-parallel to emitted photon 616.

In 306, as shown in the example of FIG. 1, Eve detects backflash 118from first photodetector 112 with eavesdropping receiver 106, which caninclude second polarization optic 110 and second photodetector 114.Eavesdropping receiver 106 can include second polarization optic 110,second photodetector 114, and optical circulator 134. As shown in FIG.1, for example, Eve can utilize optical circulator 134 to tap intoquantum channel 120 and siphon off backflash 118 without detection byAlice or Bob, and without disturbing polarization state 122 of emittedphoton 116.

Alternatively, as shown in the example of FIG. 6, Bob detects backflash618 from first photodetector 112 with backflash receiver 606. Backflashreceiver 606 can include second polarization optic 610, secondphotodetector 614, and backflash optical circulator 632. As shown inFIG. 6, for example, Bob can utilize backflash optical circulator 632 tosiphon off and investigate backflash 618 without disturbing polarizationstate 622 of emitted photon 616 for internal calibration of photonreceiver 604. Bob can use information from such investigation to monitorthe health and operation status of photon receiver 104. Bob can also usesuch an arrangement to intercept backflash 618 to prevent third-partydetection. Bob detects backflash 618 with second photodetector 614. Insome embodiments, backflash receiver 606 is configured to detectbackflash 618 from BB84 receiver 200 with second polarization optic 610.Second polarization optic 610 is configured to output polarizationdependence 624 of backflash 618. In some embodiments, secondpolarization optic 610 can be a plurality of first polarization optics108 including, for example, non-polarizing 50:50 beamsplitter 202,half-wave plate (π/8) 222, first polarizing beamsplitter 224, secondpolarizing beamsplitter 226, and/or optical circulator 134, or anycombination thereof. Second photodetector 614 is configured to detectbackflash 618 from the output of second polarization optic 610. In someembodiments, second photodetector 614 can be four channel single photonAPD array 212. In some embodiments, backflash optical circulator 632 canbe an optical isolator, an optical modulator, an optical filter, or somecombination thereof. Similar to BB84 receiver 200, in some embodiments,backflash receiver 606 can mimic the same optical arrangement and schemeas BB84 receiver 200.

In 308, as shown in the example of FIG. 1, Eve characterizespolarization dependence 124 of backflash 118 by utilizing secondpolarization optic 110. Eve can characterize polarization dependence 124of backflash 118 by using second polarization optic 110 to deduce theoriginal polarization state 122 of emitted photon 116. In someembodiments, eavesdropping receiver 106 may be configured to detectbackflash 118 from first photodetector 112 with second polarizationoptic 110. Second polarization optic 110 may be configured to outputpolarization dependence 124 of backflash 118. In some embodiments,second polarization optic 110 can be a plurality of first polarizationoptics 108 including, for example, non-polarizing 50:50 beamsplitter202, half-wave plate (π/8) 222, first polarizing beamsplitter 224,second polarizing beamsplitter 226, and/or optical circulator 134, orany combination thereof. Second photodetector 114 may be configured todetect backflash 118 from the output of second polarization optic 110.In some embodiments, second photodetector 114 is four channel singlephoton APD array 212. In some embodiments, optical circulator 134 can bean optical isolator, an optical modulator, and/or an optical filter, orsome combination thereof. Similar to photon receiver 104, in someembodiments, eavesdropping receiver 106 can mimic the same or similaroptical arrangement and scheme as photon receiver 104, for example, BB84receiver 200.

Alternatively, as shown in the example of FIG. 6, Bob characterizespolarization dependence 624 of backflash 618 by utilizing secondpolarization optic 610. Bob can characterize polarization dependence 624of backflash 618 by using second polarization optic 610 to deduce theoriginal polarization state 622 of emitted photon 616. In someembodiments, as shown in FIG. 6, DAQ subsystem 634 may be coupled toBB84 receiver 200 and backflash receiver 606, and may characterizepolarization dependence 624 of backflash 618. DAQ subsystem 634 measuresfirst photodetector output 628 from first photodetector 112 of BB84receiver 200 and second photodetector output 630 from secondphotodetector 614 of backflash receiver 606. DAQ subsystem 634 detectsand measures signals received by BB84 receiver 200 and backflashreceiver 606. For example, DAQ subsystem 634 can detect and measureemitted photon 616 received by BB84 receiver 200, and determinepolarization state 622 and basis (either HV basis 208 or AD basis 210)of emitted photon 616 by measuring which APD 214, 216, 218, 220 detectsa signal or time stamp of emitted photon 616 during a finite timewindow. Similarly, for example, DAQ subsystem 634 can detect and measurebackflash 618 generated by BB84 receiver 200 and collected by secondphotodetector 614, and characterize polarization dependence 624 ofbackflash 618 by configuring second polarization optic 610.

FIG. 4 illustrates backflash calibration setup 400 for characterizingbackflash 440, according to an embodiment. Backflash calibration setup400 can include BB84 receiver 200, pulse generator 402, pulsed laser406, time-correlated single photon counting (TCSPC) system 404, opticalfilter 418, second polarizer 426, backflash APD 428, and free-spacechannel 444. Pulse generator 402 serves as timekeeper for backflashcalibration setup 400 by setting time reference signal 436 and operatingother components of backflash calibration setup 400 on offsets set bypulse generator 402. Pulse generator 402 provides precise delays fortriggering (e.g., initiating), syncing, delaying, and gating events, andcan initiate a sequence of events and/or be triggered by an event. Forexample, offsets, such as time delay offsets, are created by pulsegenerator 402 and detected by pulsed laser 406 and/or TCSPC system 404.Detection of such offsets indicates when various events initiate, forexample, triggering an emitted laser pulse 438 laser trigger 430) and/orcollection of data from BB84 receiver 200 (e.g., BB84 receiver output432) and/or backflash APD 428 (e.g., backflash APD output 434). Pulsegenerator 402 can also define the overall time window for each datacycle, for example, for each emitted laser pulse 438 and correspondingbackflash example, a transistor-transistor logic (TTL) signal, thatinitiates some component in backflash calibration setup 400. As shown inFIG. 4, pulse generator 402 operates pulsed laser 406 by laser trigger430, and triggers (e.g., initiates) pulsed laser 406 to produce emittedlaser pulse 438. Pulse generator 402 also couples to TCSPC system 404 byduplicate laser trigger 442 and time reference signal 436. In someembodiments, pulse generator 402 can be a commercial unit (e.g.,Tektronix® AFG 3102 Dual Channel Arbitrary/Function Generator™, StanfordResearch Systems DG535 Pulse Generator™, etc.) operated at a highfrequency (e.g., 500 KHz, 1 MHz, etc.) for time reference signal 436,laser trigger 430, and duplicate laser trigger 442. In some embodiments,pulsed laser 406 can be a pulsed laser diode laser (e.g.,Opto-Electronics Inc. Model PPL1M™) producing emitted laser pulse 438 ata wavelength of about 100 nm to 1 mm, and preferably 400 nm to 1100 nm,for example, at about 850 nm.

Emitted laser pulse 438 transmits along free-space channel 444 throughoptical filter 418 to BB84 receiver 200. As described above, BB84receiver 200 detects emitted laser pulse 438, similar to emitted photon116, and sends BB84 receiver output 432 to TCSPC system 404. Backflash440 created by BB84 receiver 200 travels in the opposite direction ofemitted laser pulse 438 and is reflected off optical filter 418 androuted to second polarizer 426 and backflash APD 428. In someembodiments, due to the rotated optical filter 418, 99.9% of backflash440 is reflected off optical filter 418, while the other 0.01% ofbackflash 440 is transmitted through optical filter 418 and lost. Secondpolarizer 426 can determine correlations between the polarization ofemitted laser pulse 438 and backflash 440. Second polarizer 426 ismounted between optical filter 418 and backflash APD 428. Similar toeavesdropping receiver 106 with second photodetector 114, backflash APD428 can be a SPAR or multi-channel APD array. For example, similar tosecond photodetector 114, backflash APD 428 can be a SPAD (e.g.,Perkin-Elmer® SPCM-AQR-15-FCT™, etc.). For example, similar to secondphotodetector 114, backflash APD 428 can be four channel single photonAPD array 212 (e.g., Perkin-Elmer® SPCM-AQ4C™, etc.). Backflash APD 428detects backflash 440 and polarization dependence 124 of backflash 440,similar to backflash 118, and sends backflash APD output 434 to TCSPCsystem 404 which characterizes polarization dependence 124 of backflash440.

In some embodiments, second polarizer 426 can be adjustable linearpolarizer 446. In some embodiments, second polarizer 426 can include anactuator or a transducer for incrementally rotating second polarizer 426about its axis in increments of fractions of a degree. For example,adjustable linear polarizer 446 can be mounted within a rotator (e.g.,ThorLabs® Model PRM1Z8 Motorized Precision Rotation Mount™, etc.) forcomputerized control of the angle of adjustable linear polarizer 446with respect to the horizontal. In some embodiments, as shown in FIG. 4,optical filter 418 can be, for example, a neutral density (ND) opticalfilter (e.g., 3.0 ND, 30 ND, etc.). In some embodiments, optical filter418 can be rotated off the beam axis. For example, optical filter 418can be rotated 20° off the beam axis.

In some embodiments, data acquisition is handled by TCSPC system 404.TCSPC system 404 is coupled to pulse generator 402, BB84 receiver 200,and backflash APD 428, Pulse generator 402 provides time referencesignal 436 to synchronize (SYNC) the internal clock of TCSPC system 404.Pulse generator 402 also sends duplicate laser trigger 442 with anadvance of, for example, 1 microsecond, relative to laser trigger 430 toprovide a signal to SYNC Channel. SYNC Channel synchronizes andinitiates data collection by TCSPC system 404, and operates as anelectrical timing reference channel to compare with laser trigger 430,emitted laser pulse 438, BB84 receiver output 432, backflash 440, andbackflash APD 434 signals. For example, SYNC Channel can permitreal-time scanning of an ordered stream of recorded timing events. BB84receiver output 432 is sent to TCSPC system 404, which corresponds tothe output of four channel APD array 212. For example, first APD 214corresponds to Channel 1, second APD 216 corresponds to Channel 2, thirdAPD 218 corresponds to Channel 3, and fourth APD 220 corresponds toChannel 4. TCSPC system 404 records the start of each laser pulse cycleand the global time of every event in every channel. For example, whenAPD 214, 216, 218, 220 detects emitted laser pulse 438 or backflash APD428 detects backflash 440, respective BB84 receiver output 432 orbackflash APD output 434 is sent to TCSPC system 404, which time tagsthe signal as an event in that corresponding channel. In someembodiments, TCSPC system 404 can be a commercial unit (e.g., PicoQuant®HydraHarp 400™, etc.) operated with 1 picosecond resolution, forexample.

In some embodiments, backflash calibration setup 400 can includevariable attenuator 408. For example, as shown in FIG. 4, variableattenuator 408 can reduce the power (P) of pulsed laser 406 such thatemitted laser pulse 438 is a faint pulse, for example, P∝10⁻¹⁵ W or 1femtowatt, that approximates a single photon source. In someembodiments, backflash calibration setup 400 can include firstcollimator 410 and second collimator 412. For example, as shown in FIG.4, first and second collimators 410, 412 can be disposed at oppositeends of free-space channel 444. In some embodiments, backflashcalibration setup 400 can include backflash mirror 422. For example, asshown in FIG. 4, backflash mirror 422 can route backflash 440 fromoptical filter 418 to second polarizer 426. In some embodiments,backflash calibration setup 400 can include third collimator 424. Forexample, as shown in FIG. 4, third collimator 424 can route backflash440 transmitted through second polarizer 426 to backflash APD 428.

In some embodiments, backflash calibration setup 400 can include firstpolarizer 414 and quarter-wave plate 416. For example, as shown in FIG.4, first polarizer 414 can be a linear polarizer (π/4) 448 andquarter-wave plate 416 can be oriented such that emitted laser pulse 438is circularly polarized before transmitting through optical filter 418and impinging on BB84 receiver 200. In some embodiments, circularpolarization may be necessary for a balanced distribution of all emittedlaser pulses 438 since polarized beamsplitters evenly split circularlypolarized light. In some embodiments, backflash calibration setup 400can include beam dump 420. For example, as shown in FIG. 4, beam dump420 can collect emitted laser pulse 438 reflected off optical filter418.

FIG. 5 illustrates example backflash calibration data 500 plottingpolarization dependence 124 of backflash 440, according to anembodiment. Backflash 440 events from individual APDs 214, 216, 218, 220can be isolated by correlating BB84 receiver output 432 with backflashAPD output 434. TCSPC system 404 can analyze output data 432, 434 andplot backflash 440 events as a function of time, counts, andpolarization. In some embodiments, second polarizer 426 can beadjustable linear polarizer 446. For example, TCSPC system 404 can plotbackflash 440 as a function of rotational angle for adjustable linearpolarizer 446. Data for each backflash 440 from each APD 214, 216, 218,220 can be plotted by TCSPC system 404 to characterize polarizationdependence 124 of each backflash 440. As shown in the example of FIG. 4,adjustable linear polarizer 446 was selected for second polarizer 426for trial measurements and mounted in front of backflash APD 428. Intotal, thirteen trials were performed with adjustable linear polarizer446 rotating in 15° increments after each trial from 0° to 180°. Malus'slaw gives the relationship between the intensity of a light beam passingthrough a linear polarizer and the angle of the polarizer as:I=I ₀ cos²(θ),where I is the measured light intensity, I₀ is the incident intensity,and 0 is the angle between the direction of polarization of the incidentlight and the linear polarizer. Individual slices of backflash data canbe fit with the following generic function:A cos²(Bx+C)+D,where A is the amplitude, 2π/B is the period, C/B is the phase shift,and D is the vertical shift. One can assume B=1, since only dataassociated with linear polarizer angles of 0° to 180° providecorrelations between backflash events and polarization due to thesymmetry of adjustable linear polarizer 446.

As shown in the example of FIG. 5, data can be plotted into consecutive,non-overlapping time intervals (e.g., a slice) by binning or quantizinginto small time intervals, for example, a bin-slice of 1 ns. Estimatedphase values of five slices (e.g., each bin-slice is 1 ns wide) ofdetected backflashes 440 show four distinct phases, each correspondingto one of four APDs 214, 216, 218, 220. First APD 214 and second APD 216correspond to HV basis 208, while third APD 218 and fourth APD 220correspond to AD basis 210. Since HV basis 208 and AD basis 210correspond to two orthogonal basis states, a phase difference of π/2(˜1.57) is shown between first APD 214 and second APD 216 as well asbetween third APD 218 and fourth APD 220. Similarly, since AD basis 210is HV basis 208 rotated by π/4, a phase difference of π/4 (˜0.78) isshown between cross-basis pairs (e.g., between first APD 214 and thirdAPD 218, between second APD 216 and fourth APD 220, etc.). FIG. 5 showsa correlation between polarization dependence 124 of backflash 440 fromeach APD 214, 216, 218, 220, and the originating basis (either HV basis208 or AD basis 210) is identifiable based on the phase separation ofcross-basis pairs. As shown in FIG. 5, for any slice identified on theX-axis (horizontal time slice axis), the detection from any of APDs 214,216, 218, 220 will register as a corresponding Phase C.

FIG. 6 illustrates QKD system 600, according to some embodiments. QKDsystem 600 may include photon emitter 602, photon receiver 604, andquantum channel 620. In QKD system 600, Alice creates randomly polarizedphotons with photon emitter 602 and sends emitted photon 616 withpolarization state 622 along quantum channel 620 to Bob. Bob receivesand measures emitted photon 616 with polarization state 622 with photonreceiver 604. In some embodiments, quantum channel 620 can be free-spacechannel 648 and emitted photon 616 can have a wavelength of wavelengthof 400 nm to 1100 nm. In some embodiments, quantum channel 620 can befiber optic channel 650 and emitted photon 616 can have a wavelength of1100 nm to 1600 nm. In some embodiments, emitted photon 616 withpolarization state 622 can be circularly polarized 652.

Photon receiver 604 may include BB84 receiver 200, backflash receiver606, data acquisition (DAQ) subsystem 634, and first input port 654. Asdescribed above, BB84 receiver 200 may include first polarization optic108, first photodetector 112, and receiver input port 132. Emittedphoton 616 with polarization state 622 transmits through receiver inputport 132 and first polarization optic 108. First polarization optic 108is configured to output polarization state 622 of emitted photon 616.For example, first polarization optic 108 can be non-polarizing 50:50beamsplitter 202. In some embodiments, first polarization optic 108 canbe a plurality of first polarization optics including, for example,non-polarizing 50:50 beam splitter 202, half-wave plate (π/8) 222, firstpolarizing beamsplitter 224, and/or second polarizing beamsplitter 226,or any combination thereof. First photodetector 112 is configured todetect emitted photon 616 from the output of first polarization optic108. In some embodiments, first photodetector 112 is four channel singlephoton APD array 212. For example, four channel single photon APD array212 is configured to simultaneously detect polarization state 622 ofemitted photon 616 from photon emitter 602. As shown in FIG. 6, BB84receiver 200 generates backflash 618 traveling back out of BB84 receiver200 in a direction anti-parallel to emitted photon 616, which can bedetected by Bob with backflash receiver 606.

Backflash receiver 606 can include second polarization optic 610, secondphotodetector 614, and backflash optical circulator 632. As shown inFIG. 6, for example, Bob can utilize backflash optical circulator 632 tosiphon off and investigate backflash 618 without disturbing polarizationstate 622 of emitted photon 616 for internal calibration of photonreceiver 604. Bob detects backflash 618 with second photodetector 614and detects polarization dependence 624 of backflash 618 by using secondpolarization optic 610 to deduce the original polarization state 622 ofemitted photon 616. In some embodiments, backflash receiver 606 isconfigured to detect backflash 618 from BB84 receiver 200 with secondpolarization optic 610. Second polarization optic 610 is configured tooutput polarization dependence 624 of backflash 618. In someembodiments, second polarization optic 610 can be a plurality of firstpolarization optics 108 including, for example, non-polarizing 50:50beamsplitter 202, half-wave plate (π/8) 222, first polarizingbeamsplitter 224, second polarizing beamsplitter 226, and/or opticalcirculator 134, or any combination thereof. Second photodetector 614 isconfigured to detect backflash 618 from the output of secondpolarization optic 610. In some embodiments, second photodetector 614can be four channel single photon APD array 212. In some embodiments,backflash optical circulator 632 can be an optical isolator, an opticalmodulator, an optical filter, or some combination thereof. Similar toBB84 receiver 200, in some embodiments, backflash receiver 606 can mimicthe same optical arrangement and scheme as BB84 receiver 200.

DAQ subsystem 634 may be coupled to BB84 receiver 200 and backflashreceiver 606, and may characterize polarization dependence 624 ofbackflash 618. DAQ subsystem 634 measures first photodetector output 628from first photodetector 112 of BB84 receiver 200 and secondphotodetector output 630 from second photodetector 614 of backflashreceiver 606. DAQ subsystem 634 detects and measures signals received byBB84 receiver 200 and backflash receiver 606. For example, DAQ subsystem634 can detect and measure emitted photon 616 received by BB84 receiver200, and determine polarization state 622 and basis (either HV basis 208or AD basis 210) of emitted photon 616 by measuring which APD 214, 216,218, 220 a signal or time stamp of emitted photon 616 during a finitetime window is detected. Similarly, for example, DAQ subsystem 634 candetect and measure backflash 618 generated by BB84 receiver 200 andcollected by second photodetector 614, and determine polarizationdependence 624 of backflash 618 by configuring second polarization optic610. For example, second polarization optic 610 can be configured tomimic that of first polarization optic 108 of BB84 receiver 200 andsecond photodetector 614, similar to four channel single photon APDarray 212, can measure which APD 214, 216, 218, 220 a signal or timestamp of backflash 618 during a finite time window is detected.

In some embodiments, DAQ subsystem 634 can include pulse generator 402,TCSPC system 404, signal conditioning circuitry 656, analog-to-digitalconverters (ADC) 658, processor 660, memory 662, and/or centralprocessing unit (CPU) 664, or some combination thereof. In someembodiments, DAQ subsystem 634 is configured to decrypt the QKD orsecret key transmitted between photon emitter 602 and photon receiver604 by Alice to Bob. For example, backflash receiver 606 can determinepolarization state 622 and basis (either HV basis 208 or AD basis 210)of emitted photon 616 by measuring backflash 618 generated by BB84receiver 200 and collected by second photodetector 614, and determinepolarization dependence 624 of backflash 618 by configuring secondpolarization optic 610 to mimic that of first polarization optic 108 ofBB84 receiver 200.

In some embodiments, DAQ subsystem 634 is configured to calibrate QKDsystem 600 based on polarization dependence 624 of backflash 618. Forexample, DAQ subsystem 634 can calibrate an information leakagepercentage of QKD system 600 based on detected correlations betweenmeasured polarization state 122 of emitted photon 616 by BB84 receiver200 and corresponding backflash 618 by backflash receiver 606. In someembodiments, DAQ subsystem 634 or QKD system 600 is configured tocalibrate an external QKD system or quantum cryptography system. Forexample, the external QKD system can be calibrated by measuringpolarization dependence 624 of backflash 618 and determining aninformation leakage percentage of the external QKD system based ondetected correlations, for example, number of corresponding registeredbits (either 0 or 1) between measured polarization state 122 of emittedphoton 616 by BB84 receiver 200 and corresponding backflash 618 bybackflash receiver 606. For example, an external QKD system or quantumcryptography system can be a commercial system (e.g., ID Quantique(SwissQuantum), MagiQ Technologies, Inc. (Navajo), QuintessenceLabs(qCrypt), SeQureNet (Cygnus), etc.) or a QKD network (e.g., DARPA,SECOQC, Tokyo QKD, Los Alamos, etc.).

In some embodiments, QKD system 600 can include alarm subsystem 636. Insome embodiments, as shown in FIG. 6, alarm subsystem 636 can be coupledto DAQ subsystem 634, and trigger alarm signal 666 or alarm indicator668 if an information leakage percentage or quantum bit error rate(QBER) measured by DAQ subsystem 634 exceeds a predetermined value. Forexample, QBER can be determined by measuring the amount of destroyedstates of emitted photon 616. QBER is the ratio of an error rate to aQKD rate. QBER indicates the amount (e.g., percentage) of informationEve knows and is a measure of the secrecy between Alice and Bobrepresented by the fraction Alice's and Bob's states differ (e.g.,differences between corresponding registered bits 0 or 1 for the samepolarization basis). Since any interaction by Eve to the quantum stateswould destroy the states by perturbing the correlations (e.g.,polarization) between them, QBER increases as Eve interacts with Alice'squbits and is a simple way to check for eavesdropping. However, QBER isunaffected by backflashes, but an information leakage percentage can bemeasured. For example, the information leakage percentage can bedetermined by a backflash time correlation with DAQ subsystem 634 andBB84 receiver 200. Specifically, for example, the information leakagepercentage can be based on detected correlations of polarizationdependence 624 of backflash 618 with DAQ subsystem 634, for example,number of corresponding registered bits (either 0 or 1) between measuredpolarization state 122 of emitted photon 616 by BB84 receiver 200 andcorresponding backflash 618 by backflash receiver 606. In someembodiments, alarm indicator 668 can be visual (e.g., light, LED, etc.),audial (e.g., siren, buzzer, etc.), or vibrational. In some embodiments,as shown in FIG. 6, alarm signal 666 is coupled to and can triggerdeterring subsequent measurement mechanism 638.

In some embodiments. QKD system 600 can include deterring subsequentmeasurement mechanism 638. For example, as shown in FIG. 6, deterringsubsequent measurement mechanism 638 can include optical isolator 640,optical circulator 642, optical modulator 644, optical filter 646,optical switch 670, and/or optical router 672, or some combinationthereof. In some embodiments, backflash optical circulator 632 isoptical circulator 642 contained in BB84 receiver 200 and coupled toalarm signal 666 of alarm subsystem 636. For example, alarm signal 666can be triggered if QBER or information leakage percentage exceeds apredetermined threshold value. In some embodiments, optical filter 646can be utilized to reduce the majority of backflashes 618. For example,optical filter 646 can be a narrow-band filter with a spectral width ofabout 1 nm full width at half maximum (FWHM) centered at around emittedphoton 616 wavelength, for example, a wavelength of 850 nm. In someembodiments, optical switch 670 can be an active 2×2 optical switch. Insome embodiments, deterring subsequent measurement mechanism 638 caninclude a time delay. For example, optical switch 670 or optical router672 can include a time delay that occurs after a SYNC signal or timereference signal from photon emitter 602 is received by DAQ subsystem634 to switch off or route backflash 618 generated after emitted photon616 impinges first photodetector 112.

In some embodiments, backflash receiver 606 can be omitted from QKDsystem 600. In some embodiments, backflash optical circulator 632 can beomitted from QKD system 600. In some embodiments, backflash opticalcirculator 632 can be dynamically controlled by DAQ subsystem 634 oralarm subsystem 636. For example, alarm signal 666 can be coupled tobackflash optical circulator 632 and trigger activation of backflashoptical circulator 632 to siphon off backflash 618.

The present disclosure has been described above with the aid offunctional building blocks illustrating the implementation of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the disclosure that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent disclosure. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present disclosure should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

What is claimed is:
 1. A quantum cryptography apparatus, comprising: aphoton emitter configured to emit a photon at a wavelength; a photonreceiver coupled to the photon emitter by at least one quantum channeland comprising a first polarization optic configured to output theemitted photon in a polarization state; a first photodetector configuredto detect the emitted photon from the output of the first polarizationoptic; a second photodetector configured to detect a backflash from thefirst photodetector; and a second polarization optic between the firstphotodetector and the second photodetector, wherein the secondphotodetector and the second polarization optic are configured tointernally calibrate the photon receiver.
 2. The apparatus of claim 1,wherein the photon receiver is internally calibrated based on aninformation leakage percentage of the quantum cryptography apparatus. 3.The apparatus of claim 1, wherein the second polarization optic isconfigured to detect a polarization dependence of the backflash from thefirst photodetector.
 4. The apparatus of claim 1, wherein the secondphotodetector is a four channel single photon counting avalanchephotodiode (APD) array configured to simultaneously detect thepolarization dependence of the backflash from the first photodetector.5. The apparatus of claim 1, wherein: the at least one quantum channelis a free space channel; and the wavelength of the emitted photon is 400nm to 1100 nm.
 6. The apparatus of claim 1, wherein: the at least onequantum channel is a fiber optic channel; and the wavelength of theemitted photon is 1100 nm to 1600 nm.
 7. The apparatus of claim 1,wherein the second polarization optic comprises an adjustable linearpolarizer.
 8. A quantum key distribution system, comprising: a photonemitter configured to emit a photon at a wavelength; a photon receivercoupled to the photon emitter by at least one quantum channel andcomprising a first polarization optic configured to output the emittedphoton in a polarization state; a first photodetector configured todetect the photon emitted from the output of the first polarizationoptic; a second photodetector configured to detect a backflash from thefirst photodetector; a second polarization optic between the firstphotodetector and the second photodetector, wherein the secondphotodetector and the second polarization optic are configured tointernally calibrate the photon receiver.
 9. The system of claim 8,wherein the second polarization optic is configured to detect apolarization dependence of the backflash from the first photodetector.10. The system of claim 8, wherein: the at least one quantum channel isa free space channel; and the wavelength of the emitted photon is 400 nmto 1100 nm.
 11. The system of claim 8, wherein: the at least one quantumchannel is a fiber optic channel; and the wavelength of the emittedphoton is 1100 nm to 1600 nm.
 12. The system of claim 8, wherein thesecond photodetector is a four channel single photon counting avalanchephotodiode (APD) array configured to simultaneously detect thepolarization dependence of the backflash from the first photodetector.13. The system of claim 8, wherein the photon receiver is internallycalibrated based on an information leakage percentage of the quantum keydistribution system.
 14. The system of claim 13, wherein the informationleakage percentage is based on a number of corresponding registered bitsbetween the polarization state of the emitted photon and a polarizationdependence of the backflash.
 15. The system of claim 8, furthercomprising a data acquisition system coupled to the secondphotodetector, wherein the data acquisition system characterizes apolarization dependence of the backflash and deduces the polarizationstate of the emitted photon.
 16. The system of claim 15, wherein: thephoton receiver comprises a backflash receiver comprising an opticalcirculator, the second photodetector, and the second polarization optic;and the backflash receiver and the data acquisition system areconfigured to internally calibrate the photon receiver.
 17. The systemof claim 15, wherein the data acquisition system comprises an alarmconfigured to trigger a signal and/or an indicator.
 18. The system ofclaim 17, wherein the alarm is based on an information leakagepercentage and/or a quantum bit error rate of the quantum keydistribution system.
 19. The system of claim 15, wherein the dataacquisition system is configured to decrypt a quantum key distributionbetween the photon emitter and the photon receiver.
 20. The system ofclaim 15, wherein the data acquisition system comprises a pulsegenerator, a time-correlated single photon counting system, a signalconditioning circuit, an analog-to-digital converter, a processor, amemory, a central processing unit, or some combination thereof.